In a CRT, the distance from its deflecting center point to its screen (fluorescent screen) increases toward its periphery, so that the swing of an electron beam is the largest at four corners of the screen. Consequently, a north-south (upper-lower) pincushion distortion and an east-west (right-left) pincushion distortion are generated in an image displayed on the screen of the CRT. Particularly, the north-south pincushion distortion is referred to as an NS pincushion distortion, and the east-west horizontal pincushion distortion is referred to as an EW pincushion distortion. The larger the deflection angle of the electron beam is, the larger the pincushion distortions become.
FIG. 16(a) is a diagram showing an example of an NS pincushion distortion on a screen of a CRT, and FIG. 16(b) is a waveform diagram showing an NS pincushion distortion correction current superimposed on a vertical deflection current. In FIG. 16, H indicates a horizontal scanning period, and V indicates a vertical scanning period.
As shown in FIG. 16(a), the NS pincushion distortion on the screen of the CRT is in a shape which is constricted at its center, as compared with both its right and left ends. The NS pincushion distortion can be corrected by respectively moving the centers of horizontal scanning lines upward and downward, as indicated by arrows. Therefore, an NS pincushion distortion correction current (hereinafter abbreviated as a correction current) am which changes in a parabolic shape in the horizontal scanning period is superimposed on a sawtooth vertical deflection current VI which changes in the vertical scanning period, as shown in FIG. 16(b). The correction current am has a positive polarity in the first half of a vertical scanning interval (the upper half of the screen), and has a negative polarity in the latter half of the vertical scanning interval (the lower half of the screen). The amplitude of the correction current am increases toward upper and lower ends of the screen from the center thereof.
In order to superimpose a correction current on a vertical deflection current, a system using a supersaturated reactor and a transformer system in which a transformer is inserted in series with a vertical deflection coil and is driven by a parabolic current having a horizontal scanning period (hereinafter referred to as a horizontal parabolic current) have been conventionally employed.
FIG. 17 is a schematic view showing the correction of an NS pincushion distortion by the conventional supersaturated reactor system, where FIG. 17(a) is a diagram showing a supersaturated reactor, and FIG. 17(b) is a diagram showing the relationship between a magnetic flux density B and a magnetic field H in the supersaturated reactor.
In FIG. 17(a), a core 50 in the supersaturated reactor has three legs. Further, a core 51 is arranged on the core 50, and a permanent magnet 52 is arranged on the core 51. A horizontal deflection current HI is caused to flow through windings LH1 and LH2 of the legs on both sides of the core 50. Consequently, a magnetic flux ΦH is generated. A vertical deflection current VI is caused to flow through a winding LV of the leg at the center of the core 50. Consequently, a magnetic flux ΦV is generated. Further, a magnetic flux ΦB is generated by the permanent magnet 52. In the supersaturated reactor, when the magnetic field H is strengthened, the magnetic flux density B is saturated, as shown in FIG. 17(b).
By the configuration shown in FIG. 17(a), the correction current am is superimposed on the vertical deflection current VI supplied to the vertical deflection coil, as shown in FIG. 16(b). Also in the transformer system, the same control is carried out. In such a way, the NS pincushion distortion is corrected.
A horizontal deflection coil and a vertical deflection coil are arranged so as to be orthogonal inside a deflection yoke. From a problem in the fabrication of the deflection yoke, orthogonality between the horizontal deflection coil and the vertical deflection coil is not necessarily ensured. Accordingly, a current component caused by a horizontal deflection current is induced by electromagnetic coupling from the horizontal deflection coil to the vertical deflection coil inside the deflection yoke.
Furthermore, a horizontal flyback pulse generated in the horizontal deflection coil in a horizontal blanking interval reaches a voltage of a thousand and several hundred Vp-p (volt peak-to-peak), and a harmonic component of the horizontal flyback pulse has a frequency which is several ten times the horizontal scanning frequency. Accordingly, the horizontal deflection coil and the vertical deflection coil are coupled to each other through a stray capacitance between the horizontal deflection coil and the vertical deflection coil. Consequently, a current component caused by the horizontal deflection current is induced by electrostatic coupling from the horizontal deflection coil to the vertical deflection coil.
Induction of a current component from a horizontal deflection coil to a vertical deflection coil is referred to as HV crosstalk, and a current component induced from the horizontal deflection coil to the vertical deflection coil is referred to as an HV crosstalk component. When the HV crosstalk component is superimposed on a vertical deflection current supplied to the vertical deflection coil, scanning lines are distorted, so that an image to be displayed is distorted.
A current component caused by the vertical deflection current is induced from the vertical deflection coil to the horizontal deflection coil. However, the horizontal deflection current is as large as several ten Ap-p (ampere peak-to-peak), while the vertical deflection current is as small as 1 to 2 Ap-p. Further, a voltage of a pulse generated in the vertical deflection coil in a vertical blanking interval is less than 100 volts, and the frequency thereof is from several ten hertz to a maximum of several hundred hertz. Therefore, the current components respectively induced by electromagnetic coupling and electrostatic coupling from the vertical deflection coil to the horizontal deflection coil are so small that they are hardly worth consideration.
In the correction of the NS pincushion distortion using the conventional supersaturated reactor system and transformer system, the HV crosstalk generated inside the deflection yoke is not considered. FIG. 18 is a diagram for explaining the HV crosstalk.
FIG. 18(a) illustrates a vertical deflection current VI on which a correction current is superimposed, FIG. 18(b) illustrates a correction current am, FIG. 18(c) illustrates an HV crosstalk component CR, and FIG. 18(d) illustrates a synthesized waveform of the correction current am and the HV crosstalk component CR. In FIG. 18(a), the correction current am superimposed on the vertical deflection current VI is roughly illustrated. In FIG. 18, V indicates a vertical scanning period.
As shown in FIG. 18(a), a correction current, which changes in a parabolic shape in a horizontal scanning period, is superimposed on the sawtooth vertical deflection current VI, which changes in the vertical scanning period, in order to correct an NS pincushion distortion. The polarity of the correction current am is reversed in the upper half and the lower half of a screen of a CRT, as described above. Consequently, the correction current am superimposed on the vertical deflection current VI differs in polarity in the upper half and the lower half of the vertical deflection current VI, as shown in FIG. 18(b).
As shown in FIG. 18(c), the HV crosstalk component CR which periodically changes in a horizontal scanning periods within a vertical scanning interval is generated from a horizontal deflection coil to a vertical deflection coil. The polarity of the HC crosstalk component CR is the same within the vertical scanning interval.
When the HV crosstalk component CR is synthesized with the correction current am, as shown in FIG. 18(d), therefore, the peak of the correction current am in the first half of the vertical scanning interval is shifted to the left, and the peak of the correction current in the latter half thereof is shifted to the right. Consequently, a distortion in an image which differs in the upper half and the lower half of the screen of the CRT is generated.
Furthermore, an NS pincushion distortion generated by a combination of the deflection yoke and the CRT is ideally symmetrical. However, the NS pincushion distortion may not, in some cases, be symmetrical due to various variations in characteristics. Consequently, transverse lines may not, in some cases, be displayed straight on the screen of the CRT.
FIG. 19 is a conceptual diagram for explaining the correction of an NS pincushion distortion, where FIG. 19(a) illustrates an NS pincushion distortion at the time of uncorrection on a screen of a CRT, FIG. 19(b) illustrates a correction waveform, and FIG. 19(c) illustrates the screen of the CRT at the time of correction.
When the NS pincushion distortion shown in FIG. 19(a) is corrected using the parabolic correction waveform shown in FIG. 19(b), the NS pincushion distortion can be corrected in a linear shape, as shown in FIG. 19(c).
Meanwhile, a request to flatten the CRT is being strengthened by being affected by a recent FPD (Flat Panel Display) represented by an LCD (Liquid Crystal Display) and a PDP (Plasma Display Panel).
When the CRT is flattened, however, an NS pincushion distortion and an EW pincushion distortion are increased. The shape of the pincushion distortion on the CRT having a normal deflection angle exhibits parabolic waveform characteristics (second power (square) characteristics). However, a higher-order distortion component is generated in the pincushion distortion on the CRT having a large deflection angle such as the flattened CRT. Particularly with respect to the NS pincushion distortion, transverse lines in the horizontal direction are in a pincushion shape, causing a so-called gull-wing distortion which deviates from simple parabolic waveform characteristics (square characteristics).
FIG. 20 is a conceptual diagram for explaining the generation of a gull-wing distortion, where FIG. 20(a) illustrates an NS pincushion distortion at the time of uncorrection on a screen of a CRT, FIG. 20(b) illustrates a correction waveform, and FIG. 20(c) illustrates the screen of the CRT at the time of correction.
When the NS pincushion distortion shown in FIG. 20(a) is corrected using the parabolic correction waveform shown in FIG. 20(b), a gull-wing distortion having a high-order distortion component shown in FIG. 20(c) is generated.
FIG. 21 is a diagram showing a second power (square) waveform and a waveform having a higher-order distortion component in normalized manner. The gull-wing distortion is the difference, between the second power waveform shown in FIG. 21 and the waveform having a higher-order distortion component, generated as a distortion on the screen of the CRT.
When the deflection angle of the CRT is thus increased, the NS pincushion distortion cannot be corrected using a horizontal parabolic current having the second power (square) waveform.
A harmonic component of the horizontal parabolic current (a second power component) can be also added to a vertical deflection current. However, the inductance of a winding of the vertical deflection coil is on the order of several mH, and the resistance component of the winding of the vertical deflection coil is on the order of several ten ohms. Accordingly, the vertical deflection coil itself operates as a low-pass filter with respect to a component having a frequency which is not less than the horizontal scanning frequency. When it is considered that the harmonic component of the horizontal parabolic current is added, therefore, a harmonic component which is significantly larger than a basic horizontal parabolic current must be added to the vertical deflection current, thereby causing the necessity of widening the dynamic range of a circuit.
Furthermore, in the correction of the NS pincushion distortion using the conventional supersaturated reactor system, the horizontal parabolic current derived from of the horizontal deflection current is utilized. Accordingly, the horizontal parabolic current also flows in the vertical blanking interval, so that power consumption is high.